X-R chart is a common chart used to control process by using a variable data. Generally we require about 25 subgroup of data to create X-R chart from where we calculate the Cpk.
The sample required within each subgroup is depend on how big is your measurement confidence level, characteristic of the product and the process efficiency. Usually people use 3 – 5 sample per subgroup. This number of sample will define the constant that you need to create control limits. Read more in page 2.